The present invention relates to a device and method for preventing rollback of a vehicle on an incline. In order to prevent a vehicle from rolling back, the braking pressure is increased on at least the rear wheels, independently from the driver, if certain conditions are met.
Methods and devices for influencing the braking pressure in order to prevent vehicle movement that is not intended by the driver are known from the conventional methods and devices.
For example, a vehicle with automatic transmission can be held at a standstill by the driver, as known, using the brakes, since with the transmission engaged, the vehicle tends to move slowly forward (xe2x80x9ccreepingxe2x80x9d) due to the converter. The required driver effort can be reduced if the required braking pressure is held constant once it is applied. This can be accomplished, for example, by xe2x80x9clockingxe2x80x9d the braking pressure initiated by the driver on the wheel by a valve located between the main brake cylinder and the wheel brake cylinder as long as the vehicle is stopped. A vehicle speed sensor detects the standstill state for this purpose. The driver can then remove his foot from the brake pedal, while the wheels remain blocked by the brake. The braking pressure in the wheels is reduced as soon as the driver actuates the accelerator and thus signals his intent to start the vehicle moving. Such conventional system for creep inhibition is described in, for example, German Patent Other xe2x80x9chill holderxe2x80x9d systems are also known. These conventional systems concern the following situation: Driving off a vehicle having a manual transmission system is a complex procedure requiring the combined use of accelerator and clutch pedals in conjunction with actuating the hand brake. It is difficult to apply the correct amount of braking action, i.e., braking torque when the vehicle starts moving so that the vehicle does not roll in the wrong direction until the driving torque transmitted by the transmission is sufficient for actual motion start. There are many proposals on how to facilitate the driver""s task in this situation. In vehicles with hydraulic brake systems, for example, the wheel braking pressure can be isolated from the main brake cylinder by using a control valve. The pressure, once applied by the driver, remains even if the driver is no longer actuating the brake. This procedure is activated using a special switch. The driver can now initiate the procedure of moving the vehicle without concerning himself with the brakes. The control valve is opened as soon as vehicle motion is detected via a change in the rotary position of the drive shaft. German Patent Application No. 38 32 025 describes such a conventional hill holder starting aid.
German Patent Application No. 196 25 919 describes a system for controlling the braking action in a motor vehicle having means for setting the braking action independently of the driver""s action. Upon detecting a predefinable operating mode, in which at least the longitudinal velocity of the vehicle 0 is determined, a certain braking action is applied. Such an operating mode may be present, for example, when the driver wishes to have the aforementioned creep inhibition or the aforementioned starting aid. For this purpose, when a predefinable longitudinal vehicle speed is detected during this operating mode, the braking action is increased independently of the driver. By observing the longitudinal vehicle speed, a forward motion of the vehicle that is not desired by the driver is reliably inhibited during the operating mode (creep inhibition mode or hill holder mode). This conventional system is based on the fact that all wheel brake cylinders have the same braking pressure at the beginning of the driver-independent braking action. The situation where the braking pressure in the front and rear wheels is different is not considered.
Furthermore, methods and devices for controlling the brake system of a vehicle are known where at least the braking pressure in the wheel brakes of one rear wheel is influenced in order to distribute the braking action between at least one front wheel and one rear wheel. This influencing scheme is accomplished by setting a differential between the braking pressures of the front wheel and the rear wheel. German Patent Application No. 196 53 230 provides that the differential set between the braking pressure of the front wheel and the rear wheel is reduced when a predefinable situation is present. The predefinable situation is present when a measured quantity, representing the longitudinal vehicle speed, drops below a predefinable threshold value. As the longitudinal vehicle speed diminishes, the differential set between the braking pressure of the front wheel and the rear wheel is continuously reduced.
An object of the present invention is to improve existing devices and methods for vehicles equipped with a braking system with which, in order to distribute the braking action between at least one front wheel and one rear wheel, at least the braking pressure on the wheel brakes of a rear wheel is influenced, so that when braking action is performed on an incline, in which a suitable differential is set between the braking pressure of the front wheel and the rear wheel, the vehicle is prevented from rolling back.
The device according to the present invention prevents a vehicle from rolling back on an incline. In vehicles having a heavy rear load (caused, for example, by the vehicle cargo), which are equipped with a brake system with which, in order to distribute the braking action between at least one front wheel and one rear wheel by actuating actuators assigned to the rear wheel so that a differential is set between the brake pressure in the front wheel and the rear wheel, it may occur in the case of such braking (hereinafter referred to as EBDxe2x80x94electronic braking force distribution) on steep inclines that the braking pressure in the rear axle is insufficient for holding the vehicle on the incline after braking to a complete stop. The vehicle with a heavy rear load may then slip downward on the incline with blocked front wheels. The front axle, bearing little load, can barely transmit any braking force.
EBD braking is defined as follows: a differential is set between the braking pressure on the front wheels and the rear wheels and thus the braking action is distributed at least by actuating actuators assigned to the rear wheels of the vehicle. This distribution of the braking pressure and thus of the braking action ensures that the rear axle is not locked before the front axle. In EBD distribution the braking action is xe2x80x9clockedxe2x80x9d in the rear wheels by appropriately activating the actuators assigned to the rear wheels, i.e., the pressure remains unchanged during EBD braking and cannot be increased by the driver. On the other hand, the braking pressure of the front wheels can be increased by the driver at any time. This can be disadvantageous under certain circumstances in the case of braking a vehicle having a heavy load on an incline, namely when the braking pressure on the rear wheels is insufficient, as described above, to hold the vehicle at standstill on the incline.
The device according to the present invention has a first arrangement which determines whether the vehicle is at a standstill due to braking, in which an appropriate differential is set between the front wheel and the rear wheel, i.e., in EBD braking. Furthermore, the device according to the present invention has a second arrangement which determines whether the vehicle is rolling back from standstill. If the second arrangement detects a rollback of the vehicle, the braking pressure is increased on one rear wheel of the vehicle in order to inhibit rollback.
As long as no vehicle rollback is detected, the braking pressure that has been set is advantageously maintained at least for the rear wheels. On the other hand, the braking pressure on the front wheels can be increased by the driver.
In order to inhibit vehicle rollback, the braking pressure is advantageously increased only on the rear wheels. The braking pressure is only increased in the rear wheels because a greater braking effect can be achieved with the rear wheels in the event of rollback on an incline due to the load distribution.
Particular embodiments may be advantageous for implementing the standstill detection performed by the first arrangement and the rollback detection performed by the second arrangement.
The first embodiment is based on the evaluation of a velocity quantity, which describes the velocity of the vehicle, and the analysis of wheel speed quantities, which describe the wheel speeds of the individual wheels. Both detections according to the first embodiments operate reliably. However, due to the fact that the wheel speeds and thus also the vehicle velocity cannot be evaluated below a low characteristic velocity (the wheel speed signals generated by the rotation speed sensors are not sufficiently accurate), the vehicle, when rolling back, reaches at least this low characteristic velocity before the velocity quantity and the wheel speed quantities can be evaluated and thus before the braking pressure buildup according to the present invention can be implemented on the rear axle. Concerning this problem, the analysis, on which the second embodiment is based, of detected quantities, which show whether the alternation, characterizing the wheel speed signals, between a first and a second signal value due to the rotation characteristics of the wheel is present, represents an improvement. This alternation between the first and second signal value is present even at the lowest rotation speeds of the wheels, i.e., below the low characteristic speed. Consequently, by evaluating the detected quantities when rollback is detected, the rear axle wheel pressure buildup according to the present invention can be performed even at vehicle velocities below the low characteristic velocity.
These embodiments are based on the fact that the wheel (rotational) speed signals are signals that have been prepared in signal form. These are square signals alternating between a first and a second value.
The common feature of both embodiments is that a sensor arrangement, in particular speed sensors, is assigned to the wheels and generate wheel speed signals describing the rotation of the respective wheels. The device according to the present invention also contains an arrangement for both embodiments that determine, based on the wheel speed signals, a speed value describing the velocity of the vehicle.
According to the first embodiment, the device contains an arrangement that, based on the wheel speed signals, determines wheel (rotational) speed quantities describing the wheel speeds of the individual wheels. The wheel speed quantities are evaluated in the second arrangement to detect vehicle rollback.
According to the first embodiment, vehicle standstill is preferably defined as occurring when the velocity quantity is equal to or less than a first predefinable comparison value. Vehicle rollback is preferably defined as occurring when the wheel speed quantities of the front wheel are equal to or less than a second predefinable comparison value and the wheel speed quantity of at least one rear wheel is greater than the second predefinable comparison value.
As described above, the wheel speed signals alternate between a first and a second signal value depending on the rotation of the wheel. This alternation is evaluated in a second embodiment for standstill detection and rollback detection. According to the second embodiment, the device contains an arrangement with which the detection quantities for the individual wheels can be determined as a function of the wheel speed signals, the detection quantities alternating between the first and second signal values. These detection quantities are evaluated in the first arrangement to detect vehicle standstill and/or in the second arrangement to detect vehicle rollback.
In the second embodiment, three advantageous versions are possible for standstill detection. Vehicle standstill is advantageously defined as occurring, e.g.,
when the velocity quantity is equal to or less than a first predefinable comparison value, and when the detection quantities of the rear wheels indicate that the signal does not alternate between the first and second signal values for either of the rear wheels, or
when the velocity quantity is equal to or less than a first predefinable comparison value and when the detection values of the front wheels indicate that the signal does not alternate between the first and second signal values for either of the front wheels, or
when the velocity quantity is equal to or less than a first predefinable comparison value and when the detection values of the front wheels indicate that the signal does not alternate between the first and second signal values for either of the front wheels and when at least the detection quantity of one rear wheel indicates that the signal does not alternate between the first and second signal values.
The wording used in the third version xe2x80x9cwhen at least the detection quantity of one rear wheel indicates that the signal does not alternate between the first and second signal valuesxe2x80x9d may indicate that either one detection value shows or both detection values show at the same time that the signal does not alternate. In other words, this wording also includes a version in which vehicle standstill is occurring if, among other things, the detection values of the rear wheels show that the signal does not alternate for either of the rear wheels.
Two advantageous versions are possible for rollback detection in the second embodiment. The vehicle rollback is advantageously defined as occurring, e.g.,
when the detection quantities of the front wheels indicate that the signal does not alternate between the first and second signal values for either of the front wheels and when at least the detection quantity of one rear wheel indicates that the signal alternates between the first and second signal values, or
when the detection quantities of the front wheels indicate that the signal does not alternate between the first and second signal values for either of the front wheels and when the detection quantities of the rear wheels indicate that the signal alternates between the first and second signal value.
The braking pressure on at least one rear wheel is not increased until it is determined that vehicle rollback has been occurring for a predefined period of time. In order to determine whether or not vehicle rollback has been occurring for a predefined period of time, a time quantity, in particular a time count, is compared to a threshold value. To measure the period of time during which rollback has occurred, the time quantity is incremented by one each time vehicle rollback is determined.
Vehicle rollback after EBD braking, which results in vehicle standstill and makes it necessary to build up pressure in the rear axle, is detected more reliably by using the time quantity. Vehicle standstill with subsequent slight rollback of the vehicle is recognized in the standstill and rollback recognition according to the present invention, in particular using the detection values also in the case of a pitching vehicle during a very short standstill phase. Vehicle pitching results in a slight motion of the rear wheels, i.e., the rear axle without noticeable vehicle rollback. This is, however, detected as vehicle rollback in rollback detection, since the wheel signals alternate between a first and a second value. In order to avoid this erroneous detection, the duration of vehicle rollback is determined with the help of the time quantity. It can then be safely assumed that actual vehicle rollback is occurring only after the time quantity has exceeded a predefined time quantity threshold value, which corresponds to a predefined duration, since then it can be safely assumed that actual vehicle rollback is occurring, which causes pressure to be built up in the rear axle according to the present invention.
The advantages resulting from a combination of the evaluations serving as the basis of the two embodiments and from a combination of signals/quantities evaluated in the two embodiments is also possible.